Conventionally, it is known in the art to provide a brake apparatus for a vehicle, which automatically controls hydraulic pressure of wheel cylinders independently of an operation of a brake operating member such as a brake pedal by a driver. For example, an automatic braking device for a vehicle disclosed in JP2004-9914A includes: a master cylinder, which generates a basic hydraulic pressure (master-cylinder hydraulic pressure) based upon an operation of a vacuum booster according to the brake-pedal operation by a driver; a hydraulic pump that can generate a pressurizing hydraulic pressure higher than the basic hydraulic pressure and two normally open linear solenoid valves disposed system by system so as to control the amounts of pressurization (pressure differences) for respective systems to be applied to the basic hydraulic pressure using the pressurizing hydraulic pressure generated by the hydraulic pump.
For example, when a distance between a vehicle mounted with the above-described device and another vehicle driving ahead falls below a predetermined reference distance, this automatic braking device controls the hydraulic pump and the linear solenoid valve so as to adjust the amount of pressurization. Here, a wheel cylinder pressure is generated by adding the amount of pressurization to the basic hydraulic pressure. The hydraulic braking force is automatically controlled by applying such wheel cylinder pressure to the corresponding wheel cylinder. As a result, the vehicle is automatically applied with braking force independently of the operation of the brake-pedal operation by the driver.
As disclosed in JP2006-21745A, recent developments have led to a technology by which a regenerative cooperative braking control is implemented, which uses hydraulic braking force and a regenerative braking force by a motor. In this case, the above-described automatic braking device is applied to vehicles, which use motors as power supply, or what-is-called hybrid-electric vehicles (HEV), which use internal combustion engines and electric batteries to power electric motors.
More specifically, in the brake device disclosed in JP2006-21745A, the boosting characteristics of a vacuum booster is determined so that a basic hydraulic pressure corresponding to a depression amount against a brake pedal becomes lower than a predetermined target pressure value by a specified amount. As a result, “hydraulic braking force (basic hydraulic pressure braking force) based upon the basic hydraulic pressure (master-cylinder hydraulic pressure) can be lower than a preset target value by a specified amount.
Here, a compensation braking force is a regenerative braking force by a motor, a hydraulic braking force based upon the amounts of pressurization, or a combination thereof. A total braking force is the sum of the basic hydraulic pressure braking force and the compensation braking force. The compensation braking force, i.e., a regenerative braking force and/or a pressurizing hydraulic braking force, is adjusted according to the depression amount against the brake pedal. As a result, characteristics of the total braking force relative to the depression amount of the brake pedal then agrees with predetermined target characteristics, thereby preventing the driver from having an uncomfortable feeling.
Further, it has been recently proposed that a vacuum booster having the aforementioned boosting characteristics is applied to a vehicle, which employs only an internal combustion engine as power supply and is applied with the above-described automatic braking device. In such cases, a compensation braking force consists of only a pressurizing hydraulic braking force. That is, a total braking force is the sum of the basic hydraulic pressure braking force and the compensation braking force having only the pressurizing hydraulic braking force. The compensation braking force (pressurizing hydraulic braking force) is adjusted corresponding to the depression amount of the brake pedal in a manner that characteristics of the total braking force relative to the depression amount against the brake pedal agree with preset target characteristics.
Accordingly, it is possible to use a small-sized vacuum booster so that such vacuum booster is readily mounted on a vehicle. Further, characteristics of the total braking force relative to the depression amount of the brake pedal is designed to have more flexibilities so that various effects can be expected.
In a case where a vehicle is stationary, there is no need to generate braking force more than is required to keep the vehicle stationary. Such braking force required to keep the vehicle stationary is hereinafter referred to as a vehicle stopped state maintaining braking force. Therefore, also regarding a vehicle, which generates a total braking force being the sum of the basic hydraulic pressure braking force and the compensation braking force, when the vehicle is at the stopped state, there is no need to generate such total braking force more than is needed to keep the vehicle in a stopped state. In addition, when the vehicle is at the stopped state, a regenerative braking force is not generated. Therefore, no matter what type of vehicle it is, a motor vehicle or a hybrid vehicle, as far as the vehicle is at the stopped state, the compensation braking force is consisted of only the pressurizing hydraulic braking force, and the total braking force is consisted of only the hydraulic braking force, i.e., only the wheel cylinder pressure.
That is, as described above, as for a vehicle, which generates the total braking force being the sum of the basic hydraulic pressure braking force and the compensation braking force, when the vehicle is at the stopped state, there is no need to generate a wheel cylinder pressure, which is the sum of the basic hydraulic pressure and the amounts of pressurization (pressure differences), greater than the wheel cylinder pressure corresponding to the vehicle stopped state maintaining braking force. In other words, there is no need to increase the amounts of pressurization (pressure differences) generated by the linear solenoid valves.
Meanwhile, since the amounts of pressurization (pressure differences) are unnecessarily increased, i.e., since that a wheel cylinder pressure is unnecessarily increased, there is a possibility that various hydraulic pressure equipments, such as the linear solenoid valves, the hydraulic pumps, sealing of the wheel cylinders and so on, need to bear load. As described above, in order to reduce load subjected to those hydraulic equipments, it has been expected to prevent an unnecessary increase in a wheel cylinder pressure while a vehicle is at the stopped state.
The present invention has been made in view of the above circumstances, and provides a control unit applied to a brake apparatus for a vehicle, which generates a total braking force (=basic hydraulic pressure braking force+compensation braking force) by adding the compensation braking force to the basic hydraulic pressure braking force and prevents an unnecessary increase in a wheel cylinder pressure in a situation where the vehicle has stopped.